Helper for non-differentiable stuff

[willtebbutt]

It would be helpful to have functionality which tests that @non_differentiable stuff is properly... non-differentiable.

It looks like rand_tangent doesn't work on it at the minute, and in any case we probably want to check that it's returning zeros etc.

Relevant issue: https://github.com/JuliaDiff/ChainRules.jl/pull/398